Shortly after I opened my BRAND NEW JJ 6L6 quartete, I went to install the first tube, only to have the plastic piece in the middle of the circle of pins on the bottom break off Does this mean the tube is ruined? I can post pics if they'll help.

no the tube is not bad now. What happened is you broke the guide pin off. You need to see how the other tube fit in the base (which pins go where and make sure you put it in exactly that way in your amp.

This happened to me about a month or two ago, but I did what I just explained and I was totaly fine.